Image sensors are used to sense radiation such as light. Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) and charge-coupled device (CCD) sensors are widely used in various applications such as digital camera or mobile phone camera applications.
A typical image sensor includes various optical and electronic elements formed on a front side of the sensor. The optical elements include at least an array of individual pixels to capture light incident on the image sensor, while the electronic elements include transistors. Although the optical and electronic elements are formed on the front side, an image sensor can be operated as a frontside illuminated (FSI) image sensor or a backside illuminated (BSI) image sensor. In an FSI image sensor, light to be captured by the pixels in the pixel array is incident on the front side of the sensor, while in a BSI image sensor, the light to be captured is incident on the back side of the sensor.